Transposable elements are now thought to be common in higher organisms, and might contribute substantially to the mutation rate. Similarities in structure and behavior have been established between retroviruses and some transposable elements. Understanding the role of these elements in the biology of higher organisms is the long term objective of this research. The present focus is on a family of transposable elements in D. melanogaster known as P factors. Under certain conditions, P factors cause a syndrome of high mutability and other germline abberations known collectively as hybrid dysgenesis. The proposed experiments are aimed at determining the behavior of P elements in terms of their (a) transposition, (b) excision, and (c) regulation. Special attention will be given to the roles of these processes in the ability of P factors to invade populations. The data will also have applications for using P elements as tools in other studies of the molecular genetics of Drosophila. We approach these problems with a balanced combination of molecular and classical genetic techniques. (a) Transposition: Using in situ hybridization and other standard recombinant DNA techniques, we will follow the course of events when P factors are introduced into a genome. The phenomena of transpositional "explosions" and inter-specific invasion of P elements will also be investigated in this experiment. The insertional site specificity of P elements will be analyzed by placing a short (1000 base pairs) insertional preference region from the singed gene into a the alcohol dehydrogenase gene where P elements do not otherwise insert. (b) Excision: We will collect a large number of P element excisions and determine the spectrum of events (partial, complete, flanking, etc.) that occur. By looking for changes from small to large elements we will test the hypothesis that P elements undergo frequent gene conversion as opposed to simple excision. A continuing long term experiment will determine whether excision can result in the loss of P elements from laboratory strains. (c) Regulation: P element activity is closely regulated by tissue specificity and cytotype. We will use a hypermutable double P element insertion in the singed bristle gene and a collection of in vitro modified P elements to analyze the components in these regulation systems. The role of regulation in limiting the spread of P elements in a population will also be determined by using in situ hybridization to follow the invasion process while monitoring for suppression of transpositional activity.